The technologies in such fields as electric, electronic, office automation (OA), audio-visual (AV), and automotive industries have been making remarkable progress in recent years. Polymeric materials used in such new fields are required to have meritorious properties such as high strength and high heat resistance. They have come to be required to have high level properties such as high dimensional accuracy, strength, rigidity, solder heat resistance and thin-wall processability by the advancement of reduction of the size and wall thickness particularly of electric parts such as relay parts, coil bobbins and connectors. As one of the polymeric materials which can satisfy the requirements, aromatic polyesters are suitably used. Among the aromatic polyesters, particularly aromatic polyesters showing liquid crystalline property in molten state have desirable thin-wall processability and are rapidly coming into wide use as a material for electric parts.
However, the aromatic polyesters showing liquid crystalline property in molten state have such disadvantages as low impact resistance, low weld strength and high irregularity of shrinkage in every direction. Therefore, in fact, the improvement of these disadvantages has been strongly desired. Since these disadvantages are expected to be overcome by reducing the liquid crystalline property of the polyesters, the introduction of nucleus-substituted structure or a bending structure into the polymer chains, addition of glass fibers, etc. are now under investigation.
The aromatic polyesters showing liquid crystalline property in molten state have high heat resistance due to their stiff molecular chains. However, because of this, they have to be molded at elevated temperatures. Therefore, in some application fields wherein the reduction of heat resistance is accepted to some degree, there has been a strong requirement of developing an aromatic polyester showing liquid crystalline property in molten state which can be molded at relatively low temperatures. The introduction of a nucleus-substituted structure or a bending structure into the polymer chains has been likewise tried in order to lower the melting point of the polyesters without damaging their mechanical properties, thereby improving the moldability. When a notice is given to the introduction of a bending structure into the polymer chains, the use of resorcinol as a bending monomer is particularly of technical interest in view of its easy availability in industry.
Although acetylation method, phenyl esterification method and acid chloride method are known as the methods for producing aromatic polyesters, the aromatic polyesters showing liquid crystal property in molten state are mostly produced by acetylation method, in which polymerization is carried out by solution polymerization in a solvent having a high boiling point or by melt polymerization using substantially no solvent. In acetylation method, an aromatic hydroxy compound, one of the monomers, is converted into an acetic acid ester by the reaction between an aromatic hydroxy compound and acetic anhydride and the acetic acid ester is then polymerized by the intermolecular elimination of acetic acid. The conversion of an aromatic hydroxy compound into an acetic acid ester is generally conducted by adding acetic anhydride in an excess amount of about 1.1 moles per mole of the hydroxyl group and allowing the resulting mixture to react under reflux of acetic anhydride.
However, the preparation of resorcinol diacetate by the above method problematically accompanies the coloring of the reaction product and the formation of by-products. The purification of the colored reaction product by distillation under reduced pressure is still insufficient to obtain resorcinol diacetate having a high purity enough to use it as a monomer for the aromatic polyester. Thus, by the acetylation method, it was difficult to obtain a practically usable polymer having a resorcinol structure, sufficiently high molecular weight and good lightness and color tone, although such a polyester is expected to have desirable physical properties.
The use of the phenyl esterification method, in which polycondensation proceeds via intermolecular elimination of phenol, generally cannot raise the molecular weight of the resulting polymer so much. In addition, the size of the equipment for discharging the phenol generated during the elimination out of the system is obliged to be large due to the necessity of sufficiently keeping the temperature of the equipment at a certain level for avoiding the solidification of phenol which has such a high boiling point as 182.degree. C. This method, therefore, is industrially unapplicable.
The acid chloride method, in which polycondensation proceeds via dehydrochlorination, generally cannot raise the molecular weight of the resulting polymer so much. In addition, the use of special corrosion-resisting material is required for constructing the equipment due to the corrosive property of hydrogen chloride gas generated during the reaction. This method, therefore, is industrially unapplicable.